Question

A hollow ball completely filled with water rolls sliding without sliding on a horizontal floor. If the water freezes which of the parameter given below will remain unchanged

• A. Angular speed & Kinetic energy
• B. Linear momentum & angular momentum
• C. Angular momentum & kinetic energy
• D. Angular momentum & total energy

Answer 1

Answer: (D)

Angular momentum & total energy

Answer 2

Process of freezing is totally internal. In this process no external torque acts about the instantaneous point of contact ecause both the weight & normal contact force pass through it. Therefore the angular momentum of the ball + water remains constant. When the water freeze, it becomes solid. Therefore the M.I. of the system changes consequently the angular speed changes. Hence linear velocity & linear momentum and kinetic energy does not remain constant in its magnitude. According to the conservation of energy the total energy of the system ( surface in contact + ball + water) remains constant, but not the mechanical (Kinetic) energy. Since the centre of mass of the system (ball + water) remains at a constant height. Its gravitational energy remains constant